Semiconductor device

ABSTRACT

A semiconductor device has an antifuse element and a measurement unit. The antifuse element stores information according to whether the antifuse element is in the broken or unbroken state. The measurement unit determines a resistance value related to the resistance value of the broken antifuse element.

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2008-281595, filed on Oct. 31, 2008, thedisclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device having anantifuse element.

2. Description of Related Art

A large-capacity semiconductor memory, in particular a DRAM (DynamicRandom Access Memory) is provided with a redundant memory forimprovement of yield, so that when there exists a defective memory cell,the column or word containing the defective memory cell is replaced withthe redundant memory. The replacement with the redundant memory iscarried out by writing address information indicating the column or wordto be replaced in a ROM (Read Only Memory).

Fuse ROMs have been widely used as this type of ROMs. A fuse ROMincludes a plurality of fuse elements, and information is written byselectively cutting the fuse elements.

The cutting of the fuse elements is carried out with the use of a laserbeam. Therefore, the employment of the fuse ROMs involves variousrestrictions. For example, a large sized laser irradiation apparatus isrequired, and the writing of information is allowed only in the waferstage (before the assembly stage) of the manufacturing process.

In order to overcome these problems, there have recently been proposedROMs employing electrically breakable antifuse elements. An antifuseelement has a configuration basically identical to that of a capacitor.Specifically, an open circuit is established between the oppositeterminals of the antifuse element in its unbroken state, whereas a shortcircuit is established between the opposite terminals of the antifuseelement when a dielectric layer is broken by applying a high voltagebetween the terminals. Information is recorded according to theconductive and non-conductive states of the antifuse element.

The antifuse element can be downsized compared to a fuse element,resulting in reduction of the occupying area. Additionally, the antifuseelement enables writing (breakdown) by means of a high voltage generatedwithin the semiconductor device, eliminating the need of a large-scaledlaser irradiation apparatus. Further, the antifuse element enableswriting even after completion of the assembly process, contributing tofurther improvement of the yield. A semiconductor device having such anantifuse element is described for example in Japanese Laid-Open PatentPublication No. 2008-47215 (Patent Document 1). More specifically,Patent Document 1 describes a technique in which there is stored in anantifuse element various information, including information on defectivecells to be replaced with redundant cells, information relating to leveladjustment of an internal power generating circuit, and informationrelating to impedance adjustment of an input/output circuit.

SUMMARY

As described above, an antifuse element enables writing (breakdown) evenafter completion of the assembly process. When various information isstored in the antifuse element, as described above, it is necessary toaccurately know the resistance value of the antifuse element after itsbreakdown in order to confirm that the information has been stored.However, in a related semiconductor device having an antifuse element,it is impossible to know the resistance value of the broken antifuseelement. The present inventor has recognized that various problems areencountered if the resistance value of the antifuse element is notknown.

For example, it is possible, before an assembly process, to obtain anevaluation value corresponding to the resistance value of the brokenantifuse element by conducting measurement on a TEG (Test ElementGroup). However, the evaluation value often differs from the actualresistance value of the broken antifuse element. Further, it isimpossible to find variation in manufacture on the basis of theevaluation value.

Furthermore, after completion of the assembly process, the resistancevalue of the antifuse element cannot be measured any more since theantifuse element has been packaged.

If the resistance value of the broken antifuse element is unknown, theonly way to set the breakdown voltage conditions is to use thedetermination result whether the antifuse element is defective or not,which makes it difficult to set the conditions appropriately. Moreover,even if the antifuse element is determined to be defective, it is notpossible to identify whether the cause of the defect resides in theantifuse element itself or in other circuits such as a differentialdetermination unit.

The present invention seeks to solve one or more of the above problems,or to improve upon those problems at least in part.

In an embodiment, there is a provided a semiconductor device that has anantifuse element storing information as one of a broken state and anunbroken state thereof, and a measurement unit operative to measure aresistance value relative to a resistance value of the antifuse elementin the broken state.

The provision of the measurement unit determining a resistance valuerelated (or relative) to the resistance value of the antifuse element inits broken state makes it possible to determine the resistance valuerelated (or relative) to the resistance value of the broken antifuseelement.

BRIEF DESCRIPTION OF THE DRAWINGS

The above features and advantages of the present invention will be moreapparent from the following description of certain preferred embodimentstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram showing a principal part of a semiconductordevice according to an embodiment of the present invention;

FIG. 2 is a circuit diagram showing an example of an internalconfiguration of an antifuse (AF) circuit included in the semiconductordevice of FIG. 1; FIG. 3 is a circuit diagram showing an example of aninternal configuration of an AF determination circuit included in thesemiconductor device of FIG. 1;

FIG. 4 is a block diagram showing an example of an internalconfiguration of an AFVRF (AF Verify) circuit included in thesemiconductor device of FIG. 1;

FIG. 5 is a block diagram showing an internal configuration of an AFresistance selecting circuit included in the semiconductor device ofFIG. 1; and

FIG. 6 is a block diagram for explaining operation of a switchingcircuit included in the semiconductor device of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be now described herein with reference toillustrative embodiments. Those skilled in the art will recognize thatmany alternative embodiments can be accomplished using the teachings ofthe present invention and that the invention is not limited to theembodiments illustrated for explanatory purposes.

A semiconductor device according to the present invention is asemiconductor memory device having an antifuse element, for example, aDRAM having a redundant memory. The description below will be made onlyon the parts related to the present invention. The description of theother parts of the semiconductor device according to the invention willbe omitted since they can be configured using known techniques.

FIG. 1 is a block diagram showing a principal part of a semiconductordevice according to a first embodiment of the present invention. Asshown in FIG. 1, the semiconductor device has an AF (AntiFuse) circuit11, an AF determination circuit 12, an AFVRF (AF Verify) circuit 13, anAF resistance selecting circuit 14, and a switching circuit 15. The AFcircuit 11, the AF determination circuit 12, the AFVRF circuit 13, theAF resistance selecting circuit 14, and the switching circuit 15 performoperations as described below according to a signal issued by a controldevice 20 provided outside the semiconductor device.

The AF circuit 11 includes an antifuse element (hereafter, referred toas the AF element), and generates and outputs a detection voltagecorresponding to the state of the AF element. This means that the AFcircuit 11 functions as a detection voltage generating unit forgenerating a detection voltage. Detection voltages (AFLV) generated bythe AF circuit 11 include a first detection voltage for determiningwhether the AF element is in the broken state or in the unbroken state(during the normal operating mode), and a second detection voltage formeasuring the resistance value of the AF element in the broken state(during the AF element breakdown resistance value measurement testmode).

The AF determination circuit 12 compares the detection voltage AFLVreceived from the AF circuit 11 with a reference voltage, and outputs acomparison result. This means that the AF determination circuit 12functions as a comparator. The reference voltage is defined by adetermination voltage AFVRF from the AFVRF circuit 13 or a measurementvoltage MLV from the AF resistance selecting circuit 14, as describedbelow.

The AFVRF circuit 13 generates and outputs a determination voltage AFVRFwhich is used for determining whether the AF element is in the brokenstate or in the unbroken state. This means that the AFVRF circuit 13functions as a determination voltage generating unit.

The AF resistance selecting circuit 14 generates and outputs ameasurement voltage MLV which is used for measuring the resistance valueof the AF element in the broken state. This means that the AF resistanceselecting circuit 14 functions as a measurement voltage generating unit.The AF resistance selecting circuit 14 is capable of generating aplurality of different measurement voltages, and the AF resistanceselecting circuit 14 outputs one measurement voltage MLV that isselectively generated according to an external command.

The switching circuit 15 functions as a switching unit which outputs, asthe reference voltage, either the determination voltage AFVRF from theAFVRF circuit 13 or the measurement voltage MLV from the AF resistanceselecting circuit 14 to the AF determination circuit 12.

Operation of the semiconductor device shown in FIG. 1 will be described.

When determining whether the AF element is in the broken or unbrokenstate, the switching circuit 15 selects the determination voltage AFVRFfrom the AFVRF circuit 13, and supplies the determination voltage AFVRFto the AF determination circuit 12 as the reference voltage. The AFdetermination circuit 12 compares the detection voltage AFLV receivedfrom the AF circuit 11 with the determination voltage AFVRF as thereference voltage. When the detection voltage AFLV is higher than thereference voltage, the AF determination circuit 12 outputs adetermination result indicating that the AF element is in the unbrokenstate. When the detection voltage AFLV is lower than the referencevoltage, the AF determination circuit 12 outputs a determination resultindicating that the AF element is in the broken state. The determinationresult is supplied to a redundant control circuit in a memory circuit(not shown).

When measuring the resistance value of the AF element, the switchingcircuit 15 selects the measurement voltage MLV received from the AFresistance selecting circuit 14, and supplies the measurement voltageMLV to the AF determination circuit 12. The AF determination circuit 12compares the detection voltage AFLV from the AF circuit 11 with themeasurement voltage MLV as the reference voltage, and outputs acomparison result. The comparison result is transferred to a datainput/output terminal to which data to be written in or read from thememory circuit is supplied.

This means that, during the AF element breakdown resistance valuemeasurement test mode, an electrical path is formed between the outputof the AF determination circuit 12 and the data input/output terminal(not shown).

When measuring the resistance value of the AF element, the measurementvoltage MLV generated by the AF resistance selecting circuit 14 ischanged (increased or decreased) stepwise, while the detection voltageAFLV is compared with each of the measurement voltages MLV as thereference voltage. According to the shown embodiment, as describedabove, the measurement voltages MLV are given to the AF determinationcircuit 12 as candidate voltages for the detection voltage AFLV.

When the AF determination circuit 12 determines that the measurementvoltage MLV is changed from a value lower (or higher) than the detectionvoltage AFLV to a higher (or lower) value, the comparison result alsovaries. Therefore, the relationship between the values of themeasurement voltages MLV and the resistance values of the resistances(candidate resistance values) of the AF resistance selecting circuit 14can be previously determined so that the resistance value of the AFelement can be obtained based on the comparison result by the AFdetermination circuit 12. In other words, according to the presentinvention, the resistance value of the AF element in the broken statecan be specified by comparing the resistance with the candidateresistance values relating thereto.

Describing more specifically, when the detection voltage AFLV from theAF circuit 11 is compared with the measurement voltage MLV from the AFresistance selecting circuit 14 by the AF determination circuit 12, acandidate resistance value closest to the resistance value of an AFelement in the broken state included in the AF circuit 11 is determinedas the resistance value related to (e.g. relative to) the resistancevalue of the AF element in the broken state. The function of determiningthe resistance value of the AF element by using the candidate resistancevalue shall be herein referred as the measurement function. This meansthat the AF circuit 11, the AF resistance selecting circuit 14, (theswitching circuit 15), and the AF determination circuit 12 enclosed bythe dash-dot line in FIG. 1 collectively function as a measurement unitwhich determines the resistance value related to the resistance value ofthe AF element in the broken state included in the AF circuit 11.

The semiconductor device according to the present invention will bedescribed in further detail by additionally referring to FIGS. 2 to 6.

FIG. 2 is a circuit diagram showing an example of an internalconfiguration of the AF circuit 11, in which there are provided an AFelement 21, an AF control circuit 22, an N-channel MOS transistor(hereafter, abbreviated as NMOS) 23, and a P-channel MOS transistor(hereafter, abbreviated as PMOS) 24. The shown AF circuit 11 furtherincludes two PMOSs 25 and 26 for outputting a detection voltage AFLV tothe AF determination circuit 12. The AF element 21, the AF controlcircuit 22, the PMOSs 23, 25, 26, and the NMOS 24 operate based on asignal from the control device 20 shown in FIG. 1.

Although the shown AF circuit 11 has only one AF element 21, the AFcircuit 11 usually includes a plurality of AF elements. The AF element21 in this embodiment is formed of an MOS transistor (NMOS) the sourceand the drain of which are short-circuited. The MOS transistor havingshort-circuited source and drain operates in a similar manner to acapacitor. The MOS transistor, or the AF element can be made a virtualresistance element by applying a high voltage between the gate and thesource (drain) to break the gate insulation film.

The AF element is not limited to an MOS capacitor utilizing an MOStransistor structure, but may be provided, for example, by utilizing acapacitor structure in a DRAM memory cell.

AF Element Breaking Operation:

In order to break the AF element, a SVUPT line is driven to a voltagehigher than a normal power-supply voltage while a SVDWNT line is drivento a negative voltage. Then, the PMOS 23 and the NMOS 24 are controlledby selection signals N1 and N2 from the AF control circuit 22, so thateither a SVUPT potential or a VSS potential is supplied to an end (gate)N3 of the AF element 21. Specifically, when the AF element is to bebroken, low-level control signals N1 and N2 are supplied from the AFcontrol circuit 22 operating according to a signal from the controldevice 20, so that the PMOS 23 and the NMOS 24 are turned ON and OFF,respectively. When the AF element 21 is not to be broken, in contrast,both the control signals N1 and N2 are made high level, and the MOS 23and the NMOS 24 are turned OFF and ON, respectively.

During the AF element breaking operation, a read signal LOADB and a fuseselecting signal SELB supplied from the control device 20 are bothmaintained at a high level, and the PMOSs 25 and 26 are both OFF.Accordingly, when the AF element 21 is broken, a high voltage is appliedto an end of the AF element 21 while a negative voltage is applied tothe other end, whereby the gate insulation film of the NMOS forming theAF element 21 is broken and the AF element 21 becomes a virtualresistance element. Upon completing the AF element 21 breakingoperation, the SVUPT line and the SVDWNT line are returned to theirnormal levels under the control of the control device 20 (the SVUPT isreturned to a peripheral power-supply voltage level VPERI, and theSVDWNT line is returned to the VSS level).

Operation for Determining the State of AF Element 21:

In order to determine whether or not the AF element 21 is broken (toread the AF), the fuse selecting signals N1 and N2 supplied from the AFcontrol circuit 22 are caused to assume a high level and a low level,respectively. The read signal LOADB and the fuse selecting signal SELBare both caused to assume a low level by the control device 20. As aresult, the PMOS 23 and the NMOS 24 are both OFF and the PMOSs 25 and 26are both ON.

Accordingly, during the operation to determine the state of the AFelement 21, the AF element 21 is connected to the AF determinationcircuit 12 while being connected to the VSS power supply.

The operation to determine the state of the AF element 21 will befurther described by additionally and temporarily referring to the AFdetermination circuit 12 shown in FIG. 3.

While the AF circuit 11 is placed in the state as described above, theAF determination circuit 12 shown in FIG. 3 is supplied with a prechargesignal PREB from the control device 20. The precharge signal PREB isheld at a low level for a certain period of time, whereby a node N3 asan end (gate) of the AF element 21 shown in FIG. 2 is charged to theperipheral power-supply voltage VPERI via a PMOS 30 of the AFdetermination circuit 12 (see FIG. 3). After that, the precharge signalPREB is made high level. If the AF element 21 has been broken andoperates as a resistance element at this time, the voltage level of thenode N3 gradually drops toward the voltage level of the SVDWNT line(toward the VSS level). If the AF element 21 is not broken, in contrast,the voltage level of the node N3 remains at the charged level VPERI.These two different voltage levels of the node N3 are supplied to the AFdetermination circuit 12 as detection voltages (antifuse levels AFLV).

When measuring the resistance value of the AF element 21 as well, thevoltage level of the node N3 is supplied to the AF determination circuit12 as the detection voltage AFLV in the same manner as described above.

The internal configuration of the AF determination circuit 12 will bedescribed specifically by referring to FIG. 3 again. The AFdetermination circuit 12 includes, in addition to the above-mentionedPMOS 30 supplied with the precharge signal PREB, a differentialamplifier formed of NMOSs 31, 32, 33, and a latch circuit formed ofNMOSs 34, 35 and PMOSs 36, 37.

When the determination enable signal DEN becomes a high level under thecontrol of the control device 20, the AF determination circuit 12compares the detection voltage AFLV received from the AF circuit 11 withthe reference voltage received from the switching circuit 15, andoutputs determination output signals DB and DT according to a differencetherebetween.

As described in relation to FIG. 2, the AF determination circuit 12 issupplied with the detection voltage AFLV from the AF circuit 11 when theprecharge signal PREB becomes a high level. If the AF element 21 isbroken, the detection voltage AFLV decreases to the VSS level along theelapse of time, while the rate of the decrease depends on the resistancevalue of the AF element 21. This means that the detection voltage AFLVcontinues to exhibit a value according to the resistance value of the AFelement 21 until a certain period of time has elapsed after theprecharge signal PREB becomes a high level. Thus, the timing to make thedetermination enable signal DEN high is set at an appropriate timingduring when the detection voltage AFLV from the AF circuit 11 exhibits avalue according to the resistance value of the AF element 21.

When determining whether the AF element 21 is in the broken or unbrokenstate, the AF determination circuit 12 is supplied with a determinationvoltage AFVRF as a reference voltage from the AFVRF circuit 13.

FIG. 4 is a block diagram showing an example of the internalconfiguration of the AFVRF circuit 13. The AFVRF circuit 13 has a PMOS41 connected between a VPERI line and a VSS line, and a resistancedividing circuit 42. The resistance dividing circuit 42 has a plurality(n+1) of resistances (resistance elements) R1 to Rm+1 serially connectedto each other, and a level selector 43 which is connected to the mutualconnection points between these resistances and selectively outputs, asthe determination voltage AFVRF, one of the voltages AFV1, AFV2, AFV3, .. . , and AFVm at one of the connection points.

The plurality of resistances R1 to Rm+1 are set such that the voltagesAFV1 to AFVm obtained by resistance division by these resistances arediffered from each other by constant intervals (voltage differences),and the median value of these voltages is equal to a predeterminedvalue. This predetermined value is a median value between two detectionvoltage values output by the AF circuit 11 when determining whether theAF element 21 is broken or not broken, that is, a median value betweenthe voltage level (design value) expected when the AF element 21 isbroken and the voltage level (design value) expected when the AF element21 is not broken.

The level selector 43 is formed, for example, by a plurality oftransistor switches respectively connected between the mutual connectionpoints and the output terminal. Only one of these transistor switches isturned ON by applying a selection signal to the gates, or the controlterminals of the transistor switches so that one of the voltages AFV1 toAFVm is selectively output. It is previously determined which one of thevoltages AFV1 to AFVm is output, by conducting a test in the referencevoltage adjustment test mode. This test is conducted under the controlof the control device 20 (FIG. 1), such that the plurality of transistorswitches are turned ON sequentially and one at a time, and the voltagesAFV1 to AFVm obtained at the time when the transistor switches areturned ON are measured. Then, a value closest to the median valuebetween the two detection voltages (design values) that can be output bythe AF circuit 11 when determining whether the AF element 21 is brokenor not broken is employed. Information on which transistor switch isturned ON is stored in a memory using an electric fuse (not shown).

In the normal operating mode, the level selector 43 selects one of thevoltages AFV1 to AFVm based on the information stored in the memory notshown, and outputs the selected one as the determination voltage AFVRF.

Thus, in the normal operating mode, the determination output DT from theAF determination circuit 12 assumes a low level while the determinationoutput DB assumes a high level when the AF element 21 is broken. Incontrast, when the AF element 21 is not broken, the determination outputDT assumes a high level while the determination output DB assumes a lowlevel.

Operation for Measuring Resistance Value of AF Element 21:

Referring additionally to the AF resistance selecting circuit 14 shownin FIG. 5, the operation for measuring the resistance value of the AFelement 21 will be described. As shown in FIG. 5, the AF resistanceselecting circuit 14 has a PMOS 51 connected between a SVUPT line and aVSS line, a plurality of resistances (resistance elements) RAF1 to RAFn,and a resistance selector 52. The AF resistance selecting circuit 14further has a test signal decoder 53 connected to the resistanceselector 52, and the test signal decoder 53 operates according to asignal from the control device 20.

The PMOS 51 is turned ON and OFF according to a precharge signal PREB(the same signal as shown in FIG. 3). When supplied with a test codeT[X:0] from the control device 20 during the test mode operation, thetest signal decoder 53 decodes the test code and outputs the decodedcode as a decoded signal TS[n:0].

The resistance values of the plurality of resistances RAF1 to RAFn areset such that the resistance value is increased stepwise by apredetermined value each time from the minimum value to the maximumvalue. A substantially median value of these resistance values is set soas to be equal to the resistance value (design value) of the broken AFelement. Further, the number of the resistances RAF1 to RAFn is greaterthan the number of the resistances R1 to Rm+1 in the AFVRF circuit 13(see FIG. 4) (i.e. n>m+1) in order to enable identification of theresistance having a resistance value closest to that of the broken AFelement from among the resistances RAF1 to RAFn. This means that thevoltage level at the output node N5 of the AF resistance selectingcircuit 14 changes at smaller intervals than the change of the voltagelevel output from the AFVRF circuit 13.

The resistance selector 52 has a plurality of transistor switchesconnected between the resistances RAF1 to RAFn and the VSS,respectively. The resistance selector 52 selectively turns ON one of thetransistor switches connected to the resistances RAF1 to RAFn accordingto the decoded signal TS [n:0] from the test signal decoder.

When measuring the resistance value of the AF element 12, one of thetransistor switches connected to the resistances RAF1 to RAFn isselectively turned ON at the same time with or before the change of theprecharge signal PREB from a high level to a low level. Once theprecharge signal PREB changes to a low level, the voltage VPERI issupplied to an end of the resistance connected to the transistor switchthat has been selectively turned ON. This makes it possible to create,within the AF resistance selecting circuit 14, a state corresponding tothe state in which one end of the AF element 12 has been charged in theAF circuit 11. The AF resistance selecting circuit 14 s outputs, to theswitching circuit 15, a voltage level generated at one end of theresistance connected to the transistor switch which has been turned ON.This voltage level is used as a measurement voltage MLV after theprecharge signal PREB has changed to a high level. Although themeasurement voltage MLV at the output node N5 of the AF resistanceselecting circuit 14 also decreases with o the lapse of time, it doesnot pose any problem during the comparison by the AF determinationcircuit 12 since the timing of generation of the measurement voltage MLVdepends on the precharge signal PREB utilized in the AF circuit 11.

When measuring the resistance value of the AF element 21, themeasurement voltage MLV is changed by a predetermined value each time bysupplying the test code T[X:0] for sequentially turning ON theresistances RAF1 to RAFn, one at a time, and the precharge signal PREBhaving the timing corresponding thereto.

Thus, when measuring the resistance value of the AF element 21, the AFdetermination circuit 12 is supplied with the measurement voltage MLVfrom the AF resistance selecting circuit 14 as the reference voltage,and the determination outputs DB and DT of the AF determination circuit12 are read out every time the value of the measurement voltage MLV ischanged (for example, increased or decreased stepwise). Accordingly, theresistance value of the AF element 21 can be obtained as a value betweenthe resistance RAFj generating a voltage value corresponding to themeasurement voltage MLV at the time when the determination outputs DBand DT are changed and the resistance RAFi generating a voltage valuecorresponding to the measurement voltage MLV directly before the changeof the determination outputs DB and DT.

Switching Circuit 15:

The switching circuit 15 connected to the AFVRF circuit 13 and the AFresistance selecting circuit 14 is controlled by a signal TAFEN suppliedfrom the control device 20 as shown in FIG. 6. In the normal operatingmode and the reference voltage adjustment test mode, the signal TAFEN isat an inactive level, and the determination voltage AFVRF at the outputnode N4 of the AFVRF circuit 13 is supplied to the AF determinationcircuit 12 as the reference voltage. In the AF element breakdownresistance value measurement test mode, the signal TAFEN is at an activelevel, and the measurement voltage MLV at the output node N5 of the AFresistance selecting circuit 14 is supplied to the AF determinationcircuit 12 as the reference voltage.

As described above, the outputs of the AF determination circuit in thenormal operating mode are supplied to the redundant control circuit (notshown), whereas the outputs are supplied to the data input/outputterminals during the AF element breakdown resistance value measurementtest mode.

Although the present invention has been described above in conjunctionwith preferred embodiments thereof, the present invention is not limitedto the foregoing embodiments but may be modified and changed withoutdeparting from the scope and spirit of the invention.

For example, although in the embodiment described above, the pluralityof resistances in the AF resistance selecting circuit 14 aresequentially selected one at a time, the combination of combining theplurality of resistances in series and/or in parallel can be changed bymeans of switches so as to change the combined resistance value.Further, although in the embodiment described above, the AFdetermination circuit 12 is employed also for measuring the resistancevalue of the broken AF element in the test mode, a circuit dedicated forthe test mode may be separately provided. Further, in the embodimentdescribed above, the AF determination circuit 12 is also utilized formeasuring the resistance of the broken AF element in the test mode, acircuit dedicated for the test mode may be provided separately.

1. A semiconductor device comprising: an antifuse element storinginformation as one of a broken state and an unbroken state thereof; anda measurement unit operative to measure a resistance value relative to aresistance value of the antifuse element in the broken state.
 2. Thesemiconductor device as claimed in claim 1, wherein the measurement unitcomprises: a detection voltage generating unit generating a detectionvoltage according to one of a broken state and an unbroken statethereof; a measurement voltage generating unit including a plurality ofresistance elements and selectively generating one of a plurality ofmeasurement voltages each having a voltage level related to a resistancevalue of a corresponding one of the resistance elements, the measurementvoltages having different levels each other; and a comparator comparingthe detection voltage with the one of the measurement voltages.
 3. Thesemiconductor device as claimed in claim 2, further comprising: adetermination voltage generating unit generating one of a plurality ofdetermination voltages; and a switching unit selectively supplying oneof the one of the measurement voltages and the one of the determinationvoltages to the comparator, wherein the comparator compares, when theone of the measurement voltages is supplied, the detection voltage withthe one of the measurement voltages so as to measure the resistancevalue of the antifuse element being in the broken state and thecomparator compares, when the one of the determination voltages issupplied, the detection voltage with the one of the measurement voltagesso as to detect whether the antifuse element is in the broken state orin the unbroken state.
 4. The semiconductor device as claimed in claim2, wherein the measurement voltage generating unit comprises: theplurality of the resistance elements one ends of which are connected incommon; and a selection unit connected in common to the other ends ofthe plurality of the resistance elements and selecting one of theplurality of the resistance elements, the plurality of the resistanceelements including a design resistance element representing a designresistance value of the antifuse element in the broken state, and aplurality of resistance elements having resistance values different fromthat of the design resistance element.
 5. The semiconductor device asclaimed in claim 3, wherein the plurality of determination voltages arevaried stepwise at greater intervals than the intervals at which theplurality of measurement voltages are varied stepwise.
 6. Thesemiconductor device as claimed in claim 3, further comprising a memorycircuit, and an external terminal exchanging a signal between the memorycircuit and the outside of the semiconductor device, wherein: thecomparator supplied, when the one of the measurement voltages issupplied, a first compare result to the external terminal and thecomparator supplied, when the one of the determination voltages issupplied, a second compare result to the memory circuit.
 7. Asemiconductor device comprising: an AF circuit which includes anantifuse element storing information according to whether the antifuseelement is in the broken state or in the unbroken state, and generates adetection voltage according to the broken or unbroken state of theantifuse element; an AFVRF circuit which generates a determinationvoltage to be compared with the detection voltage to determine whetherthe antifuse element is in the broken state or in the unbroken state; anAF resistance selecting circuit which is capable of generating voltagescorresponding to a plurality of mutually different resistance values,and selectively generates a voltage corresponding to one of theplurality of resistance values as a measurement voltage; a switchingcircuit selecting either the determination voltage or the measurementvoltage; and an AF determination circuit comparing the determinationvoltage or the measurement voltage selected by the switching circuitwith the detection voltage.
 8. The semiconductor device as claimed inclaim 6, further comprising a memory circuit, and an external terminalexchanging a signal between the memory circuit and the outside of thesemiconductor device, wherein: in a normal mode, the AF determinationcircuit compares the determination voltage with the detection voltageand supplies a compare result to the memory circuit, and, in a testmode, the AF determination circuit compares the measurement voltage withthe detection voltage and supplies a compare result to the externalterminal.
 9. A measurement method for measuring a resistance value of anantifuse element in a semiconductor device, the antifuse is in a brokenstate thereof, comprising: forming a plurality of candidate resistanceelements in the semiconductor device, the candidate resistance elementshaving different resistance values each other; breaking down theantifuse element; and comparing with a voltage level of the antifuseelement with a voltage level of one of the candidate resistance elementsto determine the resistance value of the antifuse element.
 10. Themeasurement method as claimed in claim 9, further comprising,determining whether the antifuse element is in the broken state or inthe unbroken state before the comparing.